Broadband cup-dipole and cup-turnstile antennas

ABSTRACT

A cup-dipole antenna features two colinear monopole elements mounted close to the plane of the cup mouth or aperture and electrically connected at their inner ends to the outer conductors, respectively, of a pair of coaxial lines whose inner conductors are electrically connected together adjacent the monopole elements. The input ends of these coaxial lines are connected to the output or secondary-winding terminals of an impedance transformer constituting a lumped-circuit component with a secondary winding having an r.f. grounded center tap. The two monopole elements are thereby excited as a center-fed dipole. In addition, a shorting plate electrically interconnects the outer conductors of the coaxial lines between the dipole and the bottom of the cup in order to neutralize adverse effects of mutual coupling between currents flowing on the inside surface of the cup and on the outer conductors of the coaxial lines. In practice, each coaxial line is housed in a rigid pipe to which the outer conductor is electrically connected and which electrically and mechanically supports the associated monopole element. The cup turnstile form of the invention comprises four monopole elements which comprise two such dipoles disposed at right angles to each other with the two pairs of end-connected coaxial feed lines adapted to be energized in phase quadrature to produce a circularly polarized radiation pattern.

Unite States Patent Epis June 19, 1973 22 Filed:

[ BROADBAND CUP-DIPOLE AND CUP-TURNSTILE ANTENNAS [75] Inventor: James JJEpis, Sunnyvale, Calif.

[73] Assignee: GTE Sylvania Incorporated, Mountain View, Calif.

May 24, 1972 [21] Appl. No.: 256,357

Primary Examiner-Eli Lieberman Att0rneyNorman J. OMalley, John F. Lawler 'and ElmerJ.--Nealon [5 7] ABSTRACT A cup-dipole antenna features two colin'ear monopole elements mounted close to the plane of the cup mouth coaxial lines whose inner conductors are electrically i connected together adjacent the monopole elements. The input ends of these coaxial lines are connected to the output or secondary-winding terminals of an impedance transformer constituting a lumped-circuit component with a secondary winding having an r.f. grounded center tap. The two monopole elements are thereby excited as a center-fed dipole. In addition, a shorting plate electrically interconnects the outer conductors of the coaxial lines between the dipole and the bottom of the cup in order to neutralize adverse effects of mutual coupling between currents flowing on the inside surface of the cup and on the outer conductors of the coaxial lines. In practice, each coaxial line is housed in a rigid pipe to which the outer conductor is electrically connected and-which electrically and me chanically supports the associated monopole element. The cup turnstile form of the invention comprises four monopole elements which comprise two such dipoles disposed at right angles to each other with the two pairs of end-connected coaxial feed lines adapted-to be enerized radiation pattern.

7 Claims, 8 Drawing Figures SNEHJUFS PAIENIED-JUM x 9:915

m wE

SWHO OS 0.]. BALLVWBH HMSA PM'ENIED JUN I 91973 SHEEI t (If 5 BACKGROUND OF THE INVENTION This invention relates toantennas and more particularly to cup-dipole and cup-turnstile type antennas.

The cup-dipole antenna is well known in the art for its equality of radiation patterns in the electric (E) and magnetic (H) planes. The cup-turnstile antenna, for this reason, is capable of producing radiation which is substantially circularly polarized in all directions on the associated radiation pattern. The dipole pairs in such a cup-turnstile antenna are fed in phase quadrature. A disadvantage of the cup-dipole and cup-turnstile antennas in the past, however, had been their limited operating bandwidth. More specifically, the bandwidth over which these prior antennas operate with a maximum voltage standing wave ratio (VSWR) of 3.0 or less with respect to the characteristic impedance of standard feed lines has often been less than the bandwidths desired for receiving antenna system applications.

An object of this invention is the provision of cupdipole and cup-turnstile antennas having substantially improved operating bandwidths.

SUMMARY OF THE INVENTION Improvement of the operating bandwidths of cupdipole and cup-turnstile antennas is achieved with a unique feed arrangement in which each dipole is effectively center fed by'a pair of coaxial lines having center conductors electrically interconnected adjacent the FIG. 1' is a perspective view of a cup-dipole antenna embodying the invention;

FIG. 2 is a transverse section of the cup-dipole antenna shown in FIG. 1;

FIG. 3 is a greatly enlarged view partly in section of the coaxial feed lines at the point of connection to the monopole elements showing the interconnection of the center conductors;

FIG. 4 is an enlarged section of a modified form of feeding arrangement in which the coaxial lines are connected to balanced twin-lead input lines;

FIG. 5 depicts frequency VSWR performance curves of an antenna embodying this invention and a comparable prior art antenna, the frequency scale DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2, an embodiment of the invention is shown as a cup-dipole assembly comprising a conductive cup 11 with a bottom wall 12 and a cylindrical side wall 13, and a dipole 15 disposed lumped-circuit impedance transformer 24 to an external standard coaxial input line linking the antenna assembly to utilization apparatus R such as a receiver. Impedance transformer 24 is of the type having a pair of anti-phase outputs and is a commercially available component; this transformer serves to excite monopole elements 15a and 15b in the manner of a center-fed dipole and also transforms the average inherent impedance of the antenna from its relatively high value to the value of the characteristic impedance of coaxial input line 25. For example, the average antenna impedance may be approximately 150 ohms and that of the coaxial input line 25 may be ohms. In this case, for the reason described hereafter, the charcteristic impedance of each of coaxial lines 21 and 22 is preferably 75 ohms. Feed lines 21 and 22 extend through and coaxially of pipes 18 and 19, respectively, and have outer conductors 21a, 22a and inner conductors 21b, 22b; see FIG. 3. Outer conductors 21a and 22a are electrically connectd to pipes 18 and 19, respectively.'Thus, monopole elements 15a and 15b are electrically connected, to the outer conductors 21a and 22a, respectively, of the coaxial feed lines. Center conductors 21b and 22b extend beyond the upper ends ofthe outer conductors and pipes as shown in FIG. 3 and are electrically connected together as indicated at 26.

An alternate feed line arrangement is shown in FIG. 4 in which coaxial line center conductors 21b and 22b are directly connected to balanced twin input feed lines 27 and 28, respectively, thus eliminating the need for impedance transformer 24. Outer conductors 21a and 22b are electrically connected to pipes 18 and 19, re-

being a ratio of signal frequency to the center frespectively, and to cup bottom wall 12 as described above.

Pipes l8 and 19 are electrically interconnected by a shorting plate 29 which is spaced a distance P from the dipole axis B as shown in FIG. 2. The distance P is determined empirically in order to compensate the adverse effects of a mutual coupling between currents on the surfaces of the cup and pipes as well as to make the average inherent impedance of the antenna a pure resistance as described in more detail hereinafter. The pipes may also be further mechanically coupled together adjacent the dipole by a dielectric plate 30.

FIG. 5 shows a measured input VSWR performance curve 31 of the cup-dipole antenna illustrated in FIG. 1. Also shown in FIG. 5 is a measured input VSWR performance curve 32 of a prior cup-dipole antenna believed to possess the broadest bandwidth of comparable prior art antennas. The improvement in bandwidth resulting from the practice of the invention is evident from a comparison of these performance curves.

An explanation of the substantially improved VSWR: performance or bandwidth of the antenna embodying I of this impedance over abroad frequency is inherently high compared to the characteristic impedance of the most often used type of transmission line, i.e., SO-ohm coaxial line. FIG. 6 illustrates this fact with a plot 33 of the locus of inherent impedance Z, on a 50-ohm Smith Chart. Note that the inherent impedance corresponding to 2,, (f is Z =6 X 50 300 ohms. The average inherent impedance Z corresponds approximately to the center of the nearly circular impedance locus, i.e.,

for the antenna of FIG. 1 having the design parameters herewith described, Z,,,, R 3 X 50 150 ohms. Making Z,, a pure resistance is achieved by setting shorting plate 29 at a corresponding optimum distance P from the dipole, that is, 2, R, a proper selection of the distance P. The condition 2, R is important in the practice of this invention.

Feeding the dipole(s) in the manner'provided by this invention permits the inherent impedance Z,, to be shared in equal portions by each of coaxial lines 21 and 22. Thus, selecting ('l the characteristic impedance of each of these coaxial lines to equal R /2 and (2) an impedance-transformation-ratio equal to R /Z where Z is the characteristic impedance of the input coaxial line 25, results in achievement of the following highly desirable properties: i

a. The R,,,,'(at the dipole terminals) transforms to 2 at theinput terminal. I b. The nearly circular locus of inherent impedance approximately centered at R, at the dipole terminals, upon selecting the distance P optimurnly, transforms to a nearly circular impedance locus with center at Z,', at the input terminal; hence, reasonably small antenna VSWR characteristics over a broad and extended frequency band are achievable in the antenna embodying the: invention.

Achieving the foregoing desirable results-(a) and (b) also requires that the outer conductors of coaxial lines 21 and 22-be connected to r.f. ground at the location of impedance transformer 24. This, in 3 turn, requires that such transformer be the electrical equivalent of a lumped-circuit transformer having a center-tapped se condary winding, with center tap connected to.the outer conductors of coaxial lines 21 and 22 and the ends of the secondary wind-' ings connected to the center conductors of' the coaxial lines'2l and 22, respectively. The same'improved performance described by paragraph (b), above, is obtained in the case of a system employinga twin-lead input transmission line as shown in FIG. 4by simple selection of the characteristic impedance of the twin-lead line to be equal to R,,,,.

Dipole length 17.8 inches Characteristic impedanceof two wire line formed by pipes 18 and 19 148 ohms Diameter of each pipe 1.0 inch 'Center-to-center spacing of pipes 1.8 inch Characteristic impedance of each coaxial line 21,22 75 ohms v Length of each coaxial line 21, 2221.0 inches Impedance transformation ratio of tranformer 24 3:1

The performance of the above-identified antenna in terms of bandwidth and voltage standing wave ratio (VSWR) is shown in FIG. 5 and is summarized in Table I:

TABLE I Bandwidth expressed as ratio of high to VSWR BANDWIDTH frequencies Freq. Range Percent IUnder 4.411 2198 to 566.8 MHz 88.3 2.58:1 11 Under 3.51:1 222.9 to 560.4 MHz 86.2 2.514z1 b 224.5 to 556.2 MHz 85.1 2.48:1

in Under 3.00; 1

Another embodiment of the invention is shown in FIGS. 7 and 8 as a cup-turnstile antenna 32 comprising a conductive cup'33 similar to cup 11 and two crossed dipoles 35 and 36 with their respective axes at right angles to each other and disposed near the plane ofthe cup aperture. Dipoles 35 and 36 are constructed, supported and fed substantially identically as described above for dipole l5 and therefore like parts are indifrom transformer 24". The center conductors'of the coaxial feed lines for the respective dipoles are electrically interconnected adjacent the cup aperture by crossed extensions 38 and 39 which are axially spaced from each other as shown and preferably are equal in length.

Dipoles 35 and 36 are connected to utilization appa-v ratus (not shown) byan input coaxial line 40 through a 3- db quadrature hybrid coupler ,4l having output ports 42 and 43 connected to feed lines 25 and 25" Decoupled input port 44 is connected to either a matched termination or'to a second utilization apparatus. The radiation pattern applicable to.( measured at) such second utilization apparatus is identical to that applicable to first utilization apparatus, except the screw- 7 sense of the two circularly polarized radiation patterns Acup-dipole antenna embodying this invention and having the following dimensions and characteristics was constructed and successfully operated:

Inside diameter of cup reflector 24.2 inches Depth of cup 9.2 inches Center-line of dipole to bottom flat-surface of cup 9.8 inches are the opposite of one another. Shorting plate 29 electrically interconnects the four pipes 18, 18", 19 and 19" and dielectric supporting plate 30 connectd to the pipes adjacent the dipoles provides additional mechanical stability. 1 I

, What is claimed is:

1. An antenna comprising a conductive cup having a bottom wall and a side wall the end of said cup opposite the bottom wall being open and defining the cup aperture,- a dipole having two monopole elements disposed adjacent the plane of said aperture,

a pair of spaced coaxial lines extending within said cup parallel to the cup axis, each of said coaxial lines having an outer conductor and an inner conductor,

said outer conductors being electrically conducted to the bottom wall of said cup and to the inner ends of said monopole elements, respectively,

means for connecting said coaxial lines to utilization apparatus, and

means for electrically interconnecting said inner conductors adjacent said monopole elements.

2. The antenna according to claim 1 with a shorting plate electrically connected to said outer conductors and located intermediate said dipole and said cup bottom wall.

3. The antenna according to claim 2 with a pair of conductive pipes connected to said bottom wall and extending within said cup parallel to the cup axis, said coaxial lines extending through said pipes, respectively, the outer conductor of each coaxial line being electrically connected to the associated pipe, said monopole elements being connected to pipes, respectively, said shorting plate being directly connected to said pipes.

4. The antenna according to claim 2 in which said utilization apparatus connecting means comprises a balanced transmission line directly electrically connected to the innerconductors, respectively, of said coaxial lines.

5. The antenna according to claim 2 in which said utilization apparatusconnecting means comprises an impedance transformer and a coaxial input line between said utilization apparatus and the transformer, said transformer having two antiphase outputs connected to the inner conductors, respectively, of said pair of coaxial lines.

6. An antenna comprising a conductive cup having a bottom wall and a side wall, the end of said cup opposite the bott m wall being open and defining the cup aperture,

a pair of dipoles disposed near the plane of said aperture with the dipole axes at right angles to each other, each of said dipoles having two monopole elements,

four rigid conductive pipes connected to said bottom wall and extending within the cup parallel to the cup axis, the inner ends of said monopole elements being connected to said pipes, respectively,

means for electrically interconnecting said pipes at a predetermined location between the aperture and bottom wall of the cup, four coaxial lines extending through said pipes, re-

spectively, each of said coaxial lines having an outer conductor electrically connected to the associated pipe and each having an inner conductor,

means for electrically interconnecting the inner con ductors of each pair of coaxial lines associated with the monopole elements of each dipole adjacent said aperture, and

means for electrically energizing said coaxial lines.

7. The antenna according to claim 6 in which the inner conductors of one of said pairs of coaxial lines are electrically interconnected by one transverse extension, the inner conductors of the other of said pairs of coaxial lines being electrically interconnected by another transverse extension axially spaced from and equalin length to said one extension. 

1. An antenna comprising a conductive cup having a bottom wall and a side wall the end of said cup opposite the bottom wall being open and defining the cup aperture, a dipole having two monopole elements disposed adjacent the plane of said aperture, a pair of spaced coaxial lines extending within said cup parallel to the cup axis, each of said coaxial lines having an outer conductor and an inner conductor, said outer conductors being electrically conducted to the bottom wall of said cup and to the inner ends of said monopole elements, respectively, means for connecting said coaxial lines to utilization apparatus, and means for electrically interconnecting said inner conductors adjacent said monopole elements.
 2. The antenna according to claim 1 with a shorting plate electrically connected to said outer conductors and located intermediate said dipole and said cup bottom wall.
 3. The antenna according to claim 2 with a pair of conductive pipes connected to said bottom wall and extending within said cup parallel to the cup axis, said coaxial lines extending through said pipes, respectively, the outer conductor of each coaxial line being electrically connected to the associated pipe, said monopole elements being connected to pipes, respectively, said shorting plate being directly connected to said pipes.
 4. The antenna according to claim 2 in which said utilization apparatus connecting means comprises a balanced transmission line directly electrically connected to the inner conductors, respectively, of said coaxial lines.
 5. The antenna according to claim 2 in which said utilization apparatus connecting means comprises an impedance transformer and a coaxial input line between said utilization apparatus and the transformer, said transformer having two antiphase outputs connected to the inner conductors, respectively, of said pair of coaxial lines.
 6. An antenna comprising a conductive cup having a bottom wall and a side wall, the end of said cup opposite the bottom wall being open and defining the cup aperture, a pair of dipoles disposed near the plane of said aperture with the dipole axes at right angles to each other, each of said dipoles having two monopole elements, four rigid conductive pipes connected to said bottom wall and extending within the cup parallel to the cup axis, the inner ends of said monopole elements being connected to said pipes, respectively, means for electrically interconnecting said pipes at a predetermined location between the aperture and bottom wall of the cup, four coaxial lines extending through said pipes, respectively, each of said coaxial lines having an outer conductor electrically connected to the associated pipe and each having an inner conductor, means for electrically interconnecting the inner conductors of each pair of coaxial lines associated with the monopole elements of each dipole adjacent said aperture, and means for electrically energizing said coaxial lines.
 7. The antenna according to claim 6 in which the inner conductors of one of said pairs of coaxial lines are electrically interconnected by one transverse extension, the inner conductors of the other of said pairs of coaxial lines being electrically interconnected by another transverse extension axially spaced from and equal in length to said one extension. 